Abstract: The corpus callosum of many but not all BALB/c mice is either abnormally small or absent. The basis for the defect is hereditary, but the proportion of mice expressing the anomaly can be modified by changing the early environment. The present study investigated the effects of an F1 hybrid maternal environment, which is known to promote better fetal development than an inbred environment, on body and brain growth in BALB/c and on the incidence of the corpus callosal defect. Ovarian follicle cells from BALB/cWah (albino) mice were grafted into pigmented F1 hybrid, BALB/ cWah, and pigmented BALB female hosts of comparable age which were subsequently mated to BALB/cWah males. Groups of unoperated BALB/cWah and pigmented BALB dams were included as controls. Data from 111 litters were analyzed. Results showed that BALB mice with F1 mothers had heavier bodies at birth, weaning, and 100 days of age and heavier brains at 100 days than those with either grafted or ungrafted BALB mothers. The effects at birth were evident across all litter sizes (2 to 11), but those at weaning and 100 days were seen only in litters of more than 5 or 6. Neither the F1 hybrid maternal environment nor the grafting procedure itself appeared to influence the incidence or expression of the corpus callosal defect. About 22% of the offspring from each of the three groups (ungrafted BALB, grafted BALB, and grafted F1) showed a defective corpus callosum.